1. Field of the Invention
The invention relates to pulse generators, and more particularly to a method and apparatus for generating a stable and controllable soliton pulse train.
2. Description of the Background
High repetition rate, low timing jitter transmitters are required for ultra-fast time division multiplexed (TDM) networks. For soliton transmitters, there is one class that generates a soliton pulse train with a repetition rate of 20 GHz to 1 THz by adiabatically compressing and reshaping a sinusoidal optical input through a pulse compressing device. Adiabatic compression ensures transform-limited (unchirped) solitons. Various methods exist for generating the sinusoidal signal that is input into the pulse compressing device.
In Swanson et al""s xe2x80x9c40 GHz Pulse Train Generation Using Soliton Compression of a Mach-Zehnder Modulator output,xe2x80x9d IEEE Photonic Technol. Lett. 7(1), 114-116 (1995), a sinusoidal signal is generated by modulating continuous wave output from a 20 GHz signal generator using a Mach-Zender modulator. While a high quality sinusoidal signal is generated using the method, the method requires expensive and sophisticated components, including the 20 GHz electrical signal generator, and the modulator.
In xe2x80x9c40 GHz Soliton Train Generation Through Multisoliton Pulse Propagation in a Dispersion Varying Optical Fiber Circuit,xe2x80x9d IEEE Photonic Technol. Lett. 6(11) 1380-1382 (1994), Shipulin et al. describe a soliton pulse generator wherein a sinusoidal signal is generated by mixing (beating) two frequencies from two continuous wave lasers. The major disadvantage of the technique is that it is very difficult to lock the frequency between the two laser sources. Therefore, it is difficult to tune the output frequency of the generator to a desired frequency using the technique. Swanson et al. describe a soliton train generating method similar to that of Sipulin et al. in xe2x80x9c23-GHz and 123-GHz Soliton Pulse Generating Using Two CW Lasers and Standard Single-Mode Fiberxe2x80x9d IEEE Photonic Technol. Lett. 6(7), 796-798 (1994).
There exists a need for a method and apparatus for generating a soliton pulse train which utilizes passive and inexpensive components to generate a highly stable and controllable train of soliton pulses.
According to its major aspects and broadly stated, the present invention relates to a pulse generating method and associated circuitry which utilizes Brillouin scattering to generate a highly stable and controllable train of soliton pulses.
When Brillouin Scattering is stimulated in an optical fiber, the input signal generates acoustic waves through the process of electrostriction which in turn causes periodic modulation of the refractive index. The index grating scatters the input signal light through Bragg diffraction, and because of the Doppler shift associated with a grating moving at the acoustic velocity xcex3A, scattered light is down-shifted in frequency. Stimulating Brillouin scattering in an optical fiber results in a backward propagating signal shifted in wavelength from an incident signal by a magnitude that is essentially independent of the wavelength of the input signal.
The soliton pulse generator of the invention is formed by providing an input continuous wave, stimulating Brillouin scattering of an input wave having a frequency determined by the frequency of the input continuous wave to generate a backscattered wave, coupling a continuous wave having a frequency determined by the input continuous wave with the backscattered wave to generate a sinusoidal output signal, and then compressing the sinusoidal output to form a soliton pulse train. Because the wavelength shift of the backscattered wave is essentially independent of the input power and wavelength, coupling of the continuous wave and the backscattered and wavelength-shifted wave results in a highly stable and controllable sinusoidal optical signal at an output fiber of the device. A highly stable and controllable pulse generating circuit is provided by compressing the sinusoidal signal with use of a fiber whose dispersion decreases along its length in the direction of propogation (dispersion decreasing fiber) or with use of an alternate pulse compression technique.
A fiber in which Brillouin scattering takes place is considered a Brillouin fiber for purposes of the invention. To achieve Brillouin scattering in the Brillouin fiber, the power level of the first wave must be higher than the Brillouin threshold of the fiber. The Brillouin threshold for a length of fiber is determined by the Brillouin gain of the fiber, the effective core area of the fiber and the effective interaction length of the fiber. Preferably, the parameters of the Brillouin fiber are controlled so that the fiber features a low threshold so that a high intensity backward propagating Brillouin wave is easily attained. A low Brillouin threshold can be provided by decreasing the effective core area of the fiber, by increasing the length of the fiber, or by narrowing the acoustic energy spectrum of the fiber.
A high quality sinusoidal signal is produced at the output fiber if the continuous wave and the Brillouin wave have approximately equal intensities. The intensities of the continuous wave and the Brillouin wave can be made equal by amplifying or attenuating one of the waves, or by coupling the waves in a coupler having a coupling ratio which outputs the waves at equal intensities.
An important consideration in the design of the pulse generator is to ensure that Brillouin scattering is not stimulated in the output fiber at output of the second coupler. Unwanted Brillouin scattering in the output fiber can be avoided generally by increasing the Brillouin threshold of the output fiber, or by attenuating the power level of the output signal so that it is below the Brillouin threshold of the output fiber.
The Brillouin threshold of the output fiber can be increased to avoid unwanted Brillouin scattering by decreasing the Brillouin gain in the output fiber. A small Brillouin gain can be achieved by broadening the acoustic phonon spectrum. Spectral broadening can be accomplished by one of several methods including by way of doping process wherein nonuniformities are introduced into the output fiber, by providing an output fiber having a varying diameter, or by providing an output fiber having varying draw tension.
The Brillouin threshold of the output fiber can also be increased by increasing the effective core area of the output fiber, or decreasing the interaction length of the output fiber. A pulse generator according to the present invention can be made to generate a train of pulses having a repetition rate on the order of 10 Gbps. The repetition rate is readily tunable, by adjusting the temperature of the fiber which changes the acoustic velocity of the Brillouin fiber and the Brillouin fiber""s refractive index.
The 10 Gbps frequency can easily be increased by a factor of Nxc3x9710 Gbps, where N is an integer, by way of time division multiplexing. In time division multiplexing, thexcx9c10 Gbps pulse train is split using a 1xc3x97N coupler, each output path is encoded with data and delayed by Tb/N (where Tb is the original bit period) to interleave the pulses, and the several paths are recombined using an Nxc3x97P1 coupler.
A major feature of the invention is the generation of a stable and controllable sinusoidal signal by providing an input continuous wave, stimulating Brillouin scattering of an input wave having a frequency determined by the frequency of the input continuous wave to generate a backscattered wave, and coupling a continuous wave having a frequency determined by the input continuous wave with the backscattered wave to generate a sinusoidal output signal. The frequency of the sinusoidal output wave is the difference in frequency (speed of light/wavelength) between the continuous wave input and the backscattered wave. Because the backscattered wave will have a wavelength and frequency shift essentially independent of the input wavelength, the sinusoidal output generated at the output of the second coupler will be highly stable and controllable. Further, the highly stable and controllable sinusoidal output will be generated without use of an expensive and sophisticated signal generator.
Another feature of the invention is the compression of the sinusoidal signal with use of a dispersion decreasing fiber or by an alternative method. Compressing the sinusoidal signal provides a highly stable and controllable soliton pulse train.
Another major feature of the invention is the selection of a Brillouin fiber having a core area and length selected to provide a low Brillouin threshold power level. With a low Brillouin threshold, Brillouin scattering in the Brillouin fiber is easily attained.
Another feature of the invention is the coupling ratio of the first coupler. The coupling ratio of the first coupler is selected so that the first wave coupled into the Brillouin fiber is significantly above the Brillouin threshold, and thereby produces a backscattered wave of an intensity sufficient for coupling with the second wave.
Still another feature of the invention is the adjustment (by way of amplification, attenuation, or selection an appropriate coupling ratio) of the second wave and the Brillouin wave such that the two waves have equal intensities when they are coupled. Coupling the two waves at equal intensities produces a high quality sinusoidal output having maximum intensity contrast of the temporal interference.
Yet another feature of the invention is the doping of the dispersion decreasing fiber such that its acoustic energy spectrum is broadened. Broadening the spectral profile of the acoustic wave increases the Brillouin threshold in the dispersion decreasing fiber, and thereby prevents unwanted stimulated Brillouin scattering in the dispersion decreasing fiber.
These and other features of the present invention will become apparent to those skilled in the art from a reading of the ensuing Detailed Description of the Preferred Embodiments in connection with the referenced Drawings.